It is often desirable to locate, with a high degree of accuracy and specificity, locations in a blind area of a working surface. In particular, if it is desired to affix together two portions of a structure, where only an outside surface is visible to a work person, it is often difficult, if not impossible, to precisely and reproducibly place a fastener between the two portions. This is particularly relevant in regards to aircraft where the skin of the aircraft is placed over an internal frame structure and must be affixed thereto. Once the skin is in place, it is often very difficult to properly locate a fastener that must first go through the skin to be affixed to the internal structure of the aircraft. This situation arises in other construction and manufacturing instances as well.
One solution has been the attempt to back drill from inside the structure. That is, to have a work person physically place themselves inside the structure and then cut through the sub-structure through the skin. This, however, often creates impreciseness in the hole creation. For example, the full sized hole which is formed normal to the skin of the air craft, which is following the back drilled pilot hole, may be angular. That is because the hole formed from the inside of the skin can not be easily formed exactly normal to the skin of the aircraft. In particular the internal structures of the part may not be normal to the skin while the hole on through the outside of the skin must be normal to the skin. Furthermore, it is very hard on the work person who must crawl into the usually small areas to produce the holes.
Backmarkers are widely used in the aircraft industry to transfer holes from the understructure to the outside surface. Backmarkers consist of a long split piece of thin metal with a pin on one side and a hole on the other that are in alignment. The pin side is slipped under the skin to line up with a pilot hole, in the understructure, and a pilot hole is drilled into the outer skin. This method does not work on wide parts and thick parts. Deflection of the split plates and the difficulty of installing the device on thick parts limits the use to thin sheet metal areas near the edge of the skin.
Another method is to use a probe or locating device to determine a precise position on the skin. In particular, the device is first programmed with locations in three dimensional space. Therefore, when a surface is placed within reach of the probe, the probe can determine the location of a point which the probe touches. This, however, requires an extensive pre-programming and precise placement of the surface which is to be probed. Using such special orientation probes increases time and manufacturing costs for many applications. Also, probing the understructure before drilling has several shortcomings. When a skin is placed over a built up structure, the weight of the skin causes the structure and tooling to deform. It is possible that probed holes will move between measurements and drilling. Also, temperature changes between probing and drilling can cause the holes to not align due to growth or shrinkage to the part and differences in growth between the upper and lower surfaces. Fastener induced growth and coldworking of holes in aircraft structure can also shift positions of the holes between probing and drilling.
In aircraft construction, it is often critical to produce a hole, for fastening a portion of the airframe to another portion, within hundredths of an inch. One specific method of construction for internal airframe structure involves the use of sine wave topography on the internal structures or beams of the aircraft. To ensure a sufficiently strong connection, which will withstand the extreme stresses that an aircraft will encounter, the fastener must be placed at a peak of the sine wave. Therefore, placement of a fastener must be extremely precise to ensure that a peak is hit, rather than a valley or a portion adjacent to the peak. It is also desirable to precisely locate edges of hidden structure pieces. In this and many other applications, the precise locating of the fastener becomes critically important.